This application for patent of this invention is related to the following applications:
1. Distribution Line Switchgear Control Employing a Precision Rectifier invented by William N. LeCourt, Ser. No. 711804, McGraw-Edison Company;
2. Distribution Line Switchgear Control Including a Lockout Target invented by William N. LeCourt and Mark J. Ratzburg, Ser. No. 71011, McGraw-Edison Company.
The present invention relates to distribution line switchgear controls, and particularly relates to such switchgear controls which consume limited amounts of power in both quiescent and active states.
A network of power distribution lines must respond to normal variations of load requirements and to abnormal fault conditions for maintaining service to the greatest number of customers in an economical manner. To that end, a variety of switchgear is used to vary the interconnections among the distribution lines. By necessity much switchgear is located far from power distribution centers and must have stand alone capability. The types of switchgear employed include remotely actuated switches, sectionalizers, and closers.
Remotely actuated switches are controlled by a power distribution center and may include indicating means which signal their state or the occurrence of a fault on a distribution line. Switches may interrupt normal currents in a distribution line.
Sectionalizers are similar to remotely actuated switches but lack their ability to interrupt normal current. They are opened after an interrupting device has acted to interrupt their source.
Reclosers sense faults on a distribution line and open to interrupt faults (excessive current) a limited number of times within a short time interval. Most faults result from a temporary initiating condition, such as a branch brushing against a line. Once a fault resulting from a temporary condition is interrupted, the fault will not often reoccur until an initiating condition reoccurs. Faults resulting from a more permanent conditions such as a downed line are prevented from reoccurring by locking out or opening the contacts of a recloser until the cause of the fault is eliminated.
Switchgear should be located and coordinated in such a manner as to minimize the duration and area of power outage due to a fault condition.
Most switchgear requires considerable power for reliable operation. It is difficult to supply remotely located switchgear and switchgear controls with power sources which are both reliable, under normal and abnormal conditions, and inexpensive. Much switchgear is operated by springs which are tensioned to store energy under normal distribution line conditions. An example of spring operated switchgear is disclosed in U.S. Pat. No. 4,293,834 to Date et al. Switchgear controls likewise have some energy storage means to enable them to operate when their associated distribution line is disconnected.
Switchgear controls in a quiescent state, when they are not initiating a opening or closing of the distribution line switches, have relatively low power requirements. Typically, tens of milliamperes will suffice to keep a control reliably functioning in a quiescent state. However, when the switchgear control enters an active state to initiate opening or closing the distribution line switches, the power required rises dramatically. Typically, the control requirements will rise an order of magnitude or more. Amperes may be required in the active state. Occasionally, mechanisms which reduce the power requirements for some operations have been employed. An example of such a mechanism is a KFE Recloser manufactured by McGraw-Edison Company, assignee of the present invention, which employs a flux shift tripper to initiate the opening of distribution line switches. Increased power requirements for operating the control must be furnished by the energy storage means of the control. The energy storage means must either be maintained at a high level of storage or be very rapidly replenished.
The switchgear mechanisms which position the distribution line switches take a relatively long time to completely respond to a switchgear control signal. Those mechanisms which are oil filled take about a half second to completely respond. If the energy storage means can be maintained, or restored, to supply operating power within a half second, the switchgear control could initiate a close operation and then open immediately on the closing of distribution line switch. Restoring the switchgear control energy storage means within a half second would be considered adequate, even when the control is associated with mechanisms which respond much faster than a half second.
Controls for switchgear, which initiate movement of the mechanism, are often battery operated with the battery being charged under normal operating conditions. An example of a battery operated switchgear and control is disclosed in U.S. Pat. No. 3,116,439 to Riebs.
Some switchgear controls are operated from power capacitors which are charged by the distribution line under either normal or faulted conditions. An example of a power capacitor operated switchgear control is disclosed in U.S. Pat. No. 4,027,203 to Moran et al, which obtains its power from distribution line currents. Another example of power capacitor switchgear control is disclosed in U.S. Pat. No. 4,352,138 to Gilker which obtains its power from the voltage on the source side of the line.
Each energy storage means to provide switchgear control operating power has advantages and disadvantages. Battery operated switchgear controls are expensive and may become inoperative due to battery, or battery charging circuit, failures particularly when fault occurrences extend over a long period. A number of approaches to minimize failures exist. One approach, disclosed in U.S. Pat. No. 3,381,176 to Riebs et al, disconnects a battery after a successful closing operation or following a predetermined interval after a switch opening.
Capacitor powered switchgear controls avoid the expense of a battery but, usually will not maintain a state of charge as long as a battery and become uneconomical if the energy stored approaches that typically found in batteries. Often a single operation will severely deplete the amount of energy stored in a power capacitor of moderate size and cost. Depending on the size of the power capacitor and the loading in the active state, the voltage of the power capacitor and the control bus may fall on the order of five or more volts. If current transformers alone were used to restore operating voltage an undesirable period of unreliable control operation would result. A potential transformer on the source side of the switchgear may be employed to maintain charge on the power capacitor. However, such approaches, as disclosed in U.S. Pat. No. 4,352,138 to Gilker, require the addition of a potential transformer and in that invention two power capacitors. The second power capacitor operates to supply power in the event that the preferred potential transformer source of capacitor power and a secondary battery source both fail. Current transformers, necessarily present to monitor current in the distribution line, charge the second power capacitor at the expense of accuracy when charging occurs. The use of multiple sources of power to achieve greater reliability is not unusual and other approaches exist.
The use of current transformers to charge a power capacitor is particularly economical when the current transformer must be present to monitor distribution line current. However, there are difficulties associated with the use of the monitoring current transformers to charge a power capacitor. An additional dilemma involves the speed with which the capacitor is allowed to charge.
If the power capacitor is allowed to charge with great rapidity, sufficient current is drawn to adversely affect the accuracy of the current monitoring function of the control. A rapid charging of the second power capacitor in U.S. Pat. No. 4,352,138 to Gilker only occurs when the primary and secondary sources have failed. Additionally, U.S. Pat. No. 4,352,138 to Gilker discloses a circuit to prevent closing the distribution line switches when insufficient energy exists to allow the control to operate the switchgear to an open position.
Alternately the power capacitor may be placed in series with a sensing resistor which monitors phase currents as disclosed in U.S. Pat. No. 4,393,431 to Gilker, and U.S. Pat. No. 4,131,929 to Moran. However, the rate of charging the power capacitor is limited by the sensing resistor.
Alternately, the power capacitor may be charged and maintained at a more moderate rate with a less serious affect on the current monitoring function. However, during the period the power capacitor is being charged to an appropriate level, the control should be inactivated to prevent unreliable operation. U.S. Pat. No. 4,027,203 to Moran discloses one method of inhibiting control operation, while the power capacitor is being charged to an appropriate level. Similar dilemmas exists when the preferred source of supply for the power capacitor is either a potential transformer or a battery.
When a potential transformer is the preferred source, the designer must chose between providing a high or a low resistance path to the power capacitor. A low resistance path will rapidly charge the capacitor but results in vastly oversizing the transformer for an anticipated intermittent demand of the power capacitor. If the transformer is not oversized, an early failure may result under some conditions from overstressing the transformer to continuously supply what should be an intermittent demand. Alternately, if a high resistance path is chosen the power capacitor is unable to furnish sufficient power for reliable switchgear control operation for a longer period until the power capacitor has been charged.
If a battery is chosen as the preferred source for capacitor charging, the dilemma remains. A high resistance charging path results in longer periods of unreliable switchgear control operation while the power capacitor is being charged. A low resistance path will more rapidly exhaust the battery and require batteries of greater size.
Current transformers typically used in switchgear controls to monitor phase currents have ratios between the primary and secondary currents on the order of 1,000 to one. If 100 amperes is flowing through the primary of such a phase transformer, than one tenth of an ampere will flow through its seconday. One hundred milliamperes will in most cases be adequate to power the quiescent demands of the switchgear control.
Large capacitors on the order of 24,000 micro farads are used as power capacitors when the activating demands on the switchgear control are large. For every volt the power capacitor dips below the nominal bus voltage, the recovery time will be on the order of fifteen hundred cycles, if 100 amperes of alternating current is flowing in the distribution line. If the power capacitor relied on the current transformer alone and was five volts below nominal operating level, more than a second will elapse before the control is restored. Considering that the normal current on a distribution line may fluctuate an order of magnitude in a twenty-four hours the time to restoration may be in excess of ten seconds.
Some switchgear employ a momentary high voltage closing solenoid to close the distribution line switches. If the closing solenoid is appropriately fabricated, it may be employed to supply a low voltage power pulse during closing to the energy storage means of the switchgear control. An example of a switchgear control precharged to a predetermined energy level is provided by a type KFE control manufactured by McGraw-Edison Company the assignee of the present invention. While this type of momentary precharge entails additional components and expense, it avoids the above discussed dilemmas associated with capacitor charging speed. An alternate approach to momentary rapid capacitor charging, invented by Richard J. Moran, is disclosed in co-pending patent application Ser. No. 06/580,029, filed Feb. 14, 1984, which is owned by McGraw-Edison Company, assignee of the present invention. The Distribution Line Powered Control there disclosed is particularly suitable for use with switchgear which do not employ a momentary high voltage solenoid for closing the distribution line switches.
Remotely operated switches, as such, need not sense either current or voltage of their associated distribution lines. In contrast, both sectionalizers and reclosers usually sense at least distribution line current. An important point of distinction between them is that reclosers are designed to interrupt current, and that sectionalizers should not be opened while their distribution line switches are conducting current. Thus, a recloser control will measure current to determine, whether it should be interrupted and a sectionalizer control will sense the presence of current to determine whether the sectionalizer switches should remain closed.
Some sectionalizers, such as those manufactured by Togami Electric Mfg Co. Ltd. of Saga, Japan, like reclosers, employ trip counters which record the number of openings of the switches and lockout after a given number of operations The Togami type DM Systems have a reset function to initialize the trip counter after a predetermined period of conducting current in a manner similar to reclosers. Additionally, these type DM sectionalizers employ a reclose interval timer, which causes closing of the switches after a defined event. In these sectionalizers, the defined event is the existence of voltage in a distribution line. In a recloser, the reclose interval timer commences timing on the opening of the switch.
Given the nature of reclosers, and typically, their associated current transformers, a variety of schemes exist to provide accurate measurement of distribution line current over an extended range. Until saturation, current transformer output is basically linear, often in the ratio of 1,000 to 1 for distribution line current monitoring As a result of this ratio, one ampere will flow in the secondary of the current transformer, when 1,000 amperes of current is flowing in the primary The secondary current is induced to flow through a sensing resistor, often providing about five ohmns of resistance. The current transformers have an associated internal impedance which introduces a nonconstant loading factor over ranges commonly used. Given previously used sensing schemes, this normally results in an inability to accurately sense distribution line ground current below 5 amperes or above 400 amperes. This range of 80 times the minimum ground fault current is acceptable in the United States, but is not acceptable in some foreign countries. Accessories are available to basic controls which sense down to one ampere of ground current. The accessories have some adverse affects on the precision in sensing and have a very restricted range of ground current sensing. These previously used schemes can, of course, be inexpensively modified to move the range of accurate sensing upwards or downwards by modifying the current transformer ratio or by modifying the value of the resistance of the sensing resistor. However, attempts to broaden the range have resulted in unacceptable control expense for most non-specialized controls.
Typically, reset and reclose interval timers in reclosers have been activated by contacts associated with the distribution line switches in the switchgear. This contact actuation is economical, but has associated drawbacks. First, the contacts have to be available in the recloser switchgear limiting the connection of any given control to any given switchgear. Secondly, every connection between a switchgear and a control potentially couples noise into the control, which may cause malfunction. The use of solid state integrated circuits in switchgear controls have made them more sensitive to noise. The more recent generation of controls employing integrated circuits have, in many cases, an additional drawback, since their memories are volatile and dependent on the continued supply of adequate power.
Application Ser. No. 06/615,563, filed May 31, 1984, for a Recloser Control with Independent Memory, invented by Thomas Bray, and owned by McGraw-Edison Company, assignee of the present application, discloses a means of activating reset and reclose timing in a microprocessor based recloser control, which is not based on switchgear contact sensing. One of the disclosed means for achieving an independent memory is through the use of a latching relay, which inherently provides high noise immunity, because of the relatively high coercivity associated with a change of state of the relay.